How well can new particles interacting with neutrinos be constrained after a galactic supernova?

A supernova event in our own galaxy will result in a large number of neutrinos detected on Earth within the time-frame of a few seconds. These neutrinos will have been produced thermally with, in principle, three distinct temperatures for the electron, anti-electron and remaining heavy flavours respectively. We revisit the possibility that new MeV-mass particles $χ$ are also produced thermally during the event, which scatter with the neutrinos and alter their temperatures. Our main emphasis is on the detectability of this effect using the neutrino spectrum, given the large uncertainty on the temperature and density profiles of the stellar matter. By marginalising over the parameters of a simple analytic model for the stellar profile, we find that Super Kamiokande could place an upper limit on the scattering cross section at the level of $σ_{χν} \sim 10^{-40} \cdot (T / \mathrm{MeV})^2$ cm$^2$ for 10 MeV mass particles at 90% confidence. A direct-detection-like experiment would be less susceptible to systematic uncertainties in the neutrino production and mixing, but this would need a target mass around 100 tonnes in order to acquire enough statistics to compete with Super Kamiokande.